1. Field of the Invention
The invention relates to a backlight unit, and more particularly to a display device.
2. Description of the Related Art
FIG. 1 shows an application-specific integrated circuit (ASIC) and a backlight unit of a conventional display device. As shown in FIG. 1, a conventional display device 1 comprises an ASIC 10 and a backlight unit 11. The ASIC 10 comprises a voltage comparator 100 and a pulse width modulation (PWM) controller 101. The backlight unit 11 comprises a boost circuit 110 and two backlight elements 111 and 112. The boost circuit 110 comprises an inductor 110a, a transistor 110b, a diode 110c, a capacitor 110d, and a resistor 110e. When the backlight unit 11 is in a normal mode, a voltage source VCC, such as 3.3V, charges the inductor 110a according to a PWM signal S101 signal provided by the PWM controller 101 to raise a voltage value on a node N10, wherein the PWM signal S101 is alternately switched between a high logic state and a low logic state. The boost circuit 110 thus generates driving current Id1 according to the raised voltage value on the node N10. The backlight elements 111 and 112 are driven to emit light by the driving current Id1. It is assumed that the active voltage value of each backlight element is 2.6V-4V, wherein the backlight element emits low light at 2.6V and emits regular light at 3V-3.3V. Thus, the voltage value on a node N10 is raised to at least 6V for two backlight elements 111 and 112 to emit regular light. Moreover, the voltage comparator 100 detects a voltage value on a node N11 and compares the voltage value on a node N11 with a reference voltage value. The voltage comparator 100 generates a control signal S100 according to compared result. The PWM controller 101 receives the control signal S100 and modulates the pulse width of the PWM signal S101 according to the control signal S100.
When the backlight unit 11 is in a standby mode, the PWM signal S101 is in the low logic state for turning off the transistor 110b, and the voltage value on the node N10 is not raised sufficiently. The boost circuit 110 does not generate the appropriate driving current Id1, and the backlight elements 111 and 112 do not emit light.
Recently, a backlight unit capable of achieving brightness equivalent to two backlight elements using only one backlight element, as shown in FIG. 2. The backlight unit 11 comprises only one backlight element 111. As described above, the voltage source VCC provides 3.3V, and the active voltage value of the backlight element 111 is 2.6V-4V. When the backlight unit 11 is in a standby mode, the PWM signal S101 is in the low logic state for turning off the transistor 110b, and the voltage value on the node N10 is approximately equal to 3.3V, thus, the backlight element 11 can not stop emitting light or emit only a low light.
Thus, there are two solutions to the problem currently. As shown in FIGS. 3 and 4, in order to use only one backlight element, an additional resistor 30 or an additional diode 40, having a cross voltage value, replaces the backlight element 112. When the backlight unit 11 operates, however, the additional resistor 30 or the additional diode 40 induces additional power consumption.
A backlight unit of a display device with only one backlight element effectively turning off the backlight element and avoiding increased power consumption is desirable.